Most vehicles are equipped with a wheel-suspension system consisting of a spring suspension unit and a shock absorber unit. On motor vehicles, the spring suspension unit usually consists of either a coil spring, a leaf spring or an air spring and the shock absorber unit is usually a hydraulic shock absorber. The spring suspension unit is used to reduce the stresses on the vehicle. Shock absorbers are used to damp the oscillations of the part of the vehicle that are spring suspended. A worn or defective shock absorber has an impaired ability to damp, which means that the damping of oscillations in the vehicle is impaired or ceases completely. As the damping ability of the shock absorbers affects the vehicle's driving characteristics, an impaired damping ability leads to impaired driving characteristics and, in the worst case, the vehicle becomes unsafe to drive.
When a shock absorber is worn or defective, it is to be replaced by a new shock absorber. On cars, this is usually carried out depending upon the age of the shock absorber and/or the mileage for which the shock absorber has been used. Sometimes the replacement of shock absorbers is not carried out until the driver notices that the shock absorbers are noticeably worse, for example when the car sways an abnormal amount. For cars, these methods can be acceptable, particularly for cars that are driven with light loads. For cars that are driven with heavy loads or for heavy vehicles, such as trucks, such a method is not, however, acceptable. As the ageing of a truck's shock absorbers depends to a great extent upon the load, it is not possible to use age and/or mileage as the replacement criterion. In addition, it can be the case that the shock absorbers on one axle become worn more quickly than the shock absorbers on another axle, for example the shock absorbers can become worn more quickly on a front axle than on a rear axle.
In order to optimize the servicing costs, it is desirable not to replace the shock absorbers more frequently than necessary. At the same time, it is not desirable to wait to replace the shock absorbers until the vehicle is unsafe to drive. Therefore it would be desirable to be able to detect the condition of the shock absorbers in order to be able to determine a suitable occasion for when the shock absorbers are to be replaced.
There is currently no good method for detecting the condition of shock absorbers. A common way is to drive so far with the shock absorbers that the driving characteristics of the vehicle deteriorate noticeably, due to the impaired damping characteristics of the shock absorbers. This means that, in certain cases, the vehicle can become unsafe to drive.
Another way is to guess when the shock absorbers need to be replaced, on the basis of the mileage. For some haulage vehicles it is possible to estimate the ratio between load and mileage, for example, it is a reasonable estimate for a timber truck that approximately half the mileage is driven with a full load and half without a load. In a special case such as this, it can perhaps work satisfactorily. However, for vehicles that carry mixed haulage, this is a poor method.
A third way is to dismount the shock absorbers and test them in a test rig. This is an expensive and impractical method.
EP 0455993 describes a method for analyzing the condition of either a wheel, tire or shock absorber on a car. Depending upon which component is to be analyzed, two or more sensors are used, mounted on two different wheel axles. The two wheel axles must be excited with the same road unevenness. After signal processing, where among other things the signals from the front and rear axles are analyzed and where the difference between the signals is compared with stored values, the condition of the wheel, tire or shock absorber can be determined.
This method can perhaps work in certain cases, but it has a number of disadvantages. Firstly, it is necessary to have a plurality of sensors which is expensive and also means that the signal processing is complicated. Secondly, the method requires input values from the vehicle, for example the vehicle speed, which increases the complexity. In addition, the method is not suitable for vehicles, such as trucks, where the vehicle's load varies greatly between an empty and a fully-loaded vehicle.
EP 0223653 describes a method and a device for measuring the characteristics of a spring-suspension system on a vehicle. The spring-suspension system comprises shock absorbers and coil springs. In order to detect the damping characteristics of the shock absorbers, that is, in order to see whether the shock absorbers are in good condition, a signal is measured which represents the oscillations of the spring suspension. This signal is filtered with a band-pass filter around the 12 Hz range, which is the resonance frequency of tires for cars. The amplitude of the signal is then analyzed using a permanently set analogue hardware.
This method can also work in certain cases, particularly for vehicles with coil springs. The disadvantage is that it is adapted for coil spring systems which makes the method unnecessarily complicated. In addition, the method analyzes parasitic oscillations of the tires, which make it more or less reliable, depending upon the condition of the tires and the shock absorbers. The method is also affected by variations in the weight of the vehicle and variations in the spring constant of the coil springs, for example when the coil springs age.